Pile Design and Construction Practice, Fifth edition

Chapter 11
Ground investigations, piling
contracts, pile testing
The importance of a thorough ground investigation as an essential preliminary to piling
operations cannot be over-emphasized. Accurate and detailed descriptions of soil and rock
strata and an adequate programme of field and laboratory tests are necessary for the
engineer to design the piling system in the most favourable conditions.
Detailed descriptions of the ground conditions are also essential if the piling contractor is
to select the most appropriate equipment for pile installation, while giving prior warning of
possible difficulties when driving or drilling through obstructions in the ground.
The engineer must have assurance that the piles have been correctly designed and
installed in a sound manner without defects which might impair their bearing capacity. To
this end piling contracts must define clearly the responsibilities of the various parties, and
the installation of piles must be controlled at all stages of the operations. It will have become
evident from the earlier chapters of this book that load testing cannot be dispensed with as
a means of checking that the correct assumptions have been made in design and that the
deflections under the working load conform, within tolerable limits, to those predicted.
Load testing is also one of the most effective means of checking that the piles have been
soundly constructed.
The various aspects of ground investigations, piling contracts and specifications, control
of installation, load testing and other forms of test are discussed in the following sections of
this chapter.
11.1 Ground investigations
11.1.1 Planning the investigation
At the time when a ground investigation is planned it is not always certain that piled
foundations will be necessary. Therefore, the programme for the site work should follow the
usual pattern for a foundation investigation with boreholes that are sufficient in number to
give proper coverage of the site both laterally and in depth. If it becomes evident from the
initial boreholes that piling is required, or is an economical alternative to the use of shallow
spread foundations, then special attention should be given to ascertaining the level and
characteristics of a suitable stratum in which the piles can take their bearing. Where loaded
areas are large in extent, thus requiring piles to be arranged in large groups rather than in
isolated small clusters, the borings should be drilled to a depth of 1.5 times the width of the
group below the intended base level of the piles, or 1.5 times the width of the equivalent raft
below the base of the raft (Figure 11.1). This depth of exploration is necessary to obtain